1. Field of the Invention
The present invention relates to a method for processing an image in a printer, which can carry out a printing operation in two colors, and more particularly to a method for processing an image in a printer, which can effectively carry out a conversion operation by comparing quantities of light components for R (Red), G (Green) and B (Blue) associated with each pixel to be converted when a full color image is converted into a two-color image, and can decide conversion colors by applying a regular rule to all possible cases when the conversion colors are decided, such that an excellent conversion result can be achieved.
2. Description of the Related Art
Conventionally, an ECR (Electric Cash Register) or POS (Point-Of-Sale) system is equipped with a small-sized printer for printing a receipt. In this case, widely used printers include a dot printer for creating a shape of a character by electronically pushing an end of a wire bundle separately driven by an electromagnet and printing characters on a paper by striking an ink ribbon, and a thermal printer using a thermal paper for performing a printing operation by sensing heat generated from a head. The above-described printers can mostly perform only a monochrome (black or purple) printing operation. Recently, an improved dot printer, thermal printer and ink jet printer capable of performing a two-color printing operation have been developed.
Furthermore, a recently used small-sized printer has extended a range of use from printing a receipt for verifying a transaction history to printing a predetermined company logo, a specific image for marketing promotion or a ticket. According to this trend, colorful prints are required.
However, since the conventional printer is limited by a size requirement and can carry out a printing operation in only one color or two colors, the conventional printer cannot print a full-color logo or advertisement image as it is. For this reason, research on techniques for performing a conversion to a one or two-color image so that the conventional small-sized printer can print a multi-color logo or image is ongoing.
As one example, U.S. Pat. No. 5,740,333 discloses an image processing method and image processing apparatus which can convert an inputted color image to a two-color image so that the two-color image consisting of a specified color and a black color can be outputted by a copying machine, facsimile machine and printer. The inputted color image is separated into three colors of R (Red), G (Green) and B (Blue). In the above-described U.S. Pat. No. 5,740,333, a predetermined threshold value based on the specified color and two correction values are compared with quantities of light associated with the colors separated from the inputted color image, and one of the specified color and the black color is decided for each pixel of the inputted color image so that the inputted color image can be converted into the two-color image.
In the method of the above-described U.S. Pat. No. 5,740,333, the black color is basically used and the specified color is limited to a red or blue color. Since only two colors are compared at the color conversion operation, there is a problem in that the color conversion operation is limitedly carried out.
As other examples, U.S. Pat. Nos. 6,206,504 and 6,502,923 disclose a two-color ink jet apparatus and method in a POS (Point-Of-Sale) system. In the apparatus and method, a primary color and alternative color are decided in advance. Then, first, second and third color values separated from an inputted color image are compared with a given threshold value according to predetermined criteria. According to a result of the comparison, one of the primary color and alternative color to be outputted is decided, or no color is selected. As a result, an image consisting of a white color, primary color and alternative color is outputted.
In the method of the above-described U.S. Pat. Nos. 6,206,504 and 6,502,923, there are problems in that different results for the same image can be produced since the criteria for deciding the primary color and alternative color are unclear and a boundary area in which two colors can meet all criteria is not defined.